Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate, typically expressed in units of beats per minute (bpm). Examples of tachyarrhythmias include supraventricular tachycardias (SVT's) such as sinus tachycardia, atrial tachycardia (AT), and atrial fibrillation (AF). The most dangerous tachyarrhythmias, however, are ventricular tachyarrhythmias: ventricular tachycardia (VT) and ventricular fibrillation (VF). Ventricular rhythms occur when re-entry of a depolarizing wavefront in areas of the ventricular myocardium with different conduction characteristics becomes self-sustaining or when an excitatory focus in the ventricle usurps control of the heart rate from the sinoatrial node. The result is rapid and ineffective contraction of the ventricles out of electromechanical synchrony with the atria. Most ventricular rhythms exhibit an abnormal QRS complex in an electrocardiogram because they do not use the normal ventricular conduction system, the depolarization spreading instead from the excitatory focus or point of re-entry directly into the myocardium. Ventricular tachycardia is typically characterized by distorted QRS complexes that occur at a rapid rate, while ventricular fibrillation is diagnosed when the ventricle depolarizes in a chaotic fashion with no identifiable QRS complexes. Both ventricular tachycardia and ventricular fibrillation are hemodynamically compromising, and both can be life-threatening. Ventricular fibrillation, however, causes circulatory arrest within seconds and is the most common cause of sudden cardiac death.
Cardioversion (an electrical shock delivered to the heart synchronously with the QRS complex complex to terminate VT) and defibrillation (an electrical shock delivered without synchronization to the QRS complex to terminate VF) can be used to terminate most tachyarrhythmias, including SVT's, VT, and VF. The electric shock terminates the tachyarrhythmia by depolarizing all of the myocardium simultaneously and rendering it refractory. A class of cardiac rhythm management devices known as an implantable cardioverter defibrillator (ICD) provides this kind of therapy by delivering a shock pulse to the heart when the device detects tachyarrhythmias.
Another type of electrical therapy for tachycardia is anti-tachycardia pacing (ATP). In ventricular ATP, the ventricles are competitively paced with one or more pacing pulses in an effort to interrupt the reentrant circuit causing the tachycardia. Modem ICD's typically have ATP capability so that ATP therapy is delivered when VT is detected, while a shock pulse can be delivered to both VT and VF. Although cardioversion/defibrillation will terminate ventricular tachycardia, it consumes a large amount of stored power from the battery and results in patient discomfort owing to the high voltage of the shock pulses. It is desirable, therefore, for the ICD to use ATP to terminate a tachyarrhythmia whenever possible. Devices have therefore been programmed to use cardioversion/defibrillation shocks to terminate fibrillation and certain high rate tachycardias and to use ATP to treat lower rate tachycardias.
In most ICD's with ATP capability, VF is distinguished from VT using only rate-based criteria and no ATP therapy in the VF zone, so that ATP or shock therapy can be delivered as appropriate. The ventricular heart rate is usually measured by detection of the time between successive R waves (i.e., ventricular depolarizations). A measured ventricular rate is classified as a tachycardia when the rate is in a VT zone, defined as a range of rates above a tachycardia detection rate (TDR) but below a fibrillation detection rate (FDR). A measured ventricular rate above the FDR, on the other hand, is in the VF zone and is classified as ventricular fibrillation. In a typical device, a tachyarrhythmia with a heart rate in the tachycardia zone is treated with ATP therapy in order to avoid an unnecessary painful shock to the patient, and a defibrillation shock is delivered if the pacing fails to terminate the tachyarrhythmia. A commonly cited figure of merit is that for a patient with a normal sinus rhythm of 70 bpm, a rate of 150 bpm is considered tachycardia and a rate of over 200 bpm, fibrillation. The tachycardia detection zone boundary (i.e., the TDR) and the fibrillation detection zone boundary (i.e., the FDR) can also be determined for an individual patient by a procedure in which arrhythmias are purposely induced and then characterized based upon their rate, electrocardiogram waveforms, and response to treatment